1. Field of the Invention
This invention relates to mirrors, and to a method for making mirrors comprising applying a reflective layer such as silver to a glass surface and, in particular, to treating the silver surface to provide enhanced corrosion resistance of the silver surface and mirror without the need for a corrosion inhibiting copper layer on the silver surface.
2. Description of Related Art
For convenience, the following description will be directed to mirrors and improving the corrosion resistance of the reflective layer used to make the mirror but it will be understood to those skilled in the art that other metal containing substrates and metal particles may be treated using the method and apparatus of the invention to enhance the corrosion resistance of the metal.
Typical mirrors are made of a glass sheet and a thin layer of a reflective metallic film applied to the back of the sheet. The metallic film layer applied directly to the glass is usually a film of silver, although other metallic films may also be used, such as copper. When silver is used as the primary reflective layer, it is commonly protected by a second metallic film layer of copper to inhibit corrosion of the silver layer. A layer of paint is also typically used over the silver or copper layer for enhanced corrosion and abrasion resistance. Each step in the mirror making process is usually performed in an automated process with the glass sheet positioned horizontally as the sheet moves continuously through the various steps of the process. The time and need for each step is therefore very important from a commercial standpoint and the elimination of a step or substitution of a more efficient and environmentally acceptable step is a continuing goal of industry.
Mirrors are generally made continuously by a sequence of steps on a mirror conveyor. The first step lightly polishes and cleans the glass surface and after rinsing, the next step sensitizes the surface with an aqueous stannous chloride solution. The silver film layer is then deposited on the sensitized glass surface by one of many methods such as described in U.S. Pat. No. 4,737,188 to Bahls. Typically, an ammoniacal silver nitrate solution and a reducing agent solution containing a strong base are sprayed on and combined at the sensitized glass surface to deposit the silver film. Thereafter, a copper film may be applied to and over the silver film by any of a variety of prior art procedures such as a galvanic process which utilizes an aqueous suspension of iron powder and an aqueous solution of copper sulfate or by the disproportionation of cuprous ions on the silver surface. The latter process is described in U.S. Pat. No. 5,419,926 to Soltys . The copper layer is normally painted to produce the finished mirror, or another protective coating such as a hardened organic resin incorporating a corrosion inhibitor may likewise be applied as shown in U.S. Pat. No. 5,156,917 to Sanford. The above patents are hereby incorporated by reference. A standard mirror making process thus comprises a series of steps which steps are performed in sequence on a conveyor as part of a continuous mirror making process.
A serious problem of the mirror manufacturing industry is the need for a copper layer on the silver layer to inhibit corrosion of the silver. The application of copper to the silver surface necessarily produces copper containing waste streams which must be environmentally treated or processed for recycling. Typically, the copper streams are treated to remove copper before discharge to the effluent and this procedure is complex and costly. The copper film on the mirror is also a weak link in the life of a conventional mirror. The copper film is easily corroded when the mirror is subjected to ammonia or alkaline glass cleaners because these cause the edges of mirrors to corrode and turn black in color thereby shortening the life of the mirror.
A number of patents have been issued for improving treatment of the silver surface in the mirror making process to eliminate the copper layering step. In U.S. Pat. No. 5,374,451 to Servais et al., a mirror is shown having a reflective layer of silver which has been treated with a solution containing ions of at least one of the group consisting of: Cr (II); V (II or III); Ti (III or II); Fe (II); In (I or II); Cu (I); and Al (III). The solution may also contain Sn(II) ions. It is preferred in the patent that a protective layer of paint still be used to protect the treated silver layer. A similar treatment is disclosed in U.S. Pat. No. 5,240,776 which uses stannous ions to contact the silver layer followed by a silane treatment.
All the above patents are incorporated herein by reference.
Unfortunately, current methods of treating the silver surfaces of mirrors to inhibit corrosion thereof are unreliable, and specially developed corrosion inhibiting coatings are required, and the processes of U.S. Pat. Nos. 5,240,776 and 5,374,451, supra, only treat the silver surface with a simple metal solution to augment the metal atoms on the silver surface as stated in the 5,240,776 patent but which processes may not be entirely effective for the myriad of mirror products made in industry.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide a method for making mirrors wherein the copper layering process is replaced with an environmentally friendly process step which protects the silver layer against corrosion and which may be used in existing commercial mirror making conveyor systems without having to add extra sections to the system.
It is another object of the present invention to provide a method for enhancing the corrosion resistance of a metal surface such as the silver surface of a mirror and, additionally, to treat the silver surface of the mirror to enhance the corrosion resistance of the silver.
It is a further object of the invention to provide an apparatus for making mirrors without the need for the copper layering step.
Another object of the invention is to provide an improved corrosion inhibiting mirror and other metal substrate and metal articles of manufacture.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to those skilled in the art, are achieved in the present invention which is directed to, in a first aspect, a method for making mirrors having enhanced reflective metal, e.g., silver, corrosion resistance and to eliminate the need for a copper protective layer on the reflective layer, the method comprising:
supplying a glass substrate;
sensitizing the glass substrate by one of several prior art methods;
coating a layer of silver on the glass substrate;
contacting the silver coated glass substrate with a first solution containing a specified cation and a second solution containing a specified anion, or alkaline material which forms hydroxyl ions, preferably simultaneously by spraying or otherwise directing streams of the solutions so that the two solutions meet at the silver surface, the specified cation and specified anion or hydroxyl ion being reactive to form a water insoluble precipitate on the silver surface, which precipitate enhances the corrosion resistance of the silver layer.
The reaction of the first solution and the second solution may generally be shown by the following equation:
AB+CDxe2x86x92AD↓+BC
wherein A is the specified cation, D is the specified anion or alkaline material, AB and CD are water soluble compounds and BC is a water soluble reaction product and AD is a water insoluble reaction product precipitate of the specified cation A and specified anion or hydroxyl ion D. The ↓ indicates a precipitated compound. When the first solution AB and second solution CD are mixed, a supersaturated solution of product AD is formed and an opalescent mixture is obtained showing the presence of a precipitate.
The treated silver surface may be optionally painted or otherwise coated to provide the mirror product. Any organic-based mirror backing paint may be used, leaded or lead-free, and water-based mirror backing paint being a typical and alternate paint. Lilly Industries mirror backing paints are preferred.
In another aspect of the invention a method for enhancing the corrosion resistance of a metal coating such as the silver surface of a mirror comprises contacting the metal coating with a first solution containing a specified cation and a second solution containing a specified anion, or alkaline material which forms hydroxyl ions, preferably simultaneously by spraying or otherwise directing a stream of each solution so that the solutions meet at the metal coating surface, the specified cation and specified anion or hydroxyl ion being reactive to form a water insoluble reaction product precipitate on the metal surface to enhance the corrosion resistance of the metal surface.
In another aspect of the invention, either the first solution containing a specified cation or the second solution containing a specified anion, or alkaline material which forms hydroxyl ions, may be applied separately and then the other solution applied to form the reaction product precipitate. For this technique it is preferred that the solution first applied to the surface be maintained in a liquid form on the metal surface so that the cations and anions or hydroxyl ions of the solution are in the ionized form and suitable for reaction with the solution applied thereto to form the reaction product precipitate.
In a further aspect of the invention, an apparatus is provided for making mirrors comprising:
means for moving a sheet of glass preferably along a horizontal path;
means for cleaning the sheet of glass to remove oils, grease, powder, interleaving material, etc.;
means for sensitizing the cleaned sheet of glass by one of several known methods to promote silver deposition thereon;
means for applying a layer of silver on the sensitized glass surface;
means for contacting a first solution containing a specified cation and a second solution containing a specified anion, or alkaline material which forms hydroxyl ions, at the silver surface, preferably simultaneously, the specified cation ion and specified anion or alkaline material reacting to form a water insoluble reaction product precipitate on the silver surface;
means for optionally applying a silane treatment over the precipitate by techniques known to those familiar with the art; and
means for optionally painting or applying another protective layer on the precipitate to form the mirror product.
In another aspect, an improved mirror and other metal substrate and metal articles of manufacture are provided made by the method and/or apparatus of the invention.
In another aspect of the invention, the precipitated protective layer can comprise more than one substance such as a hydroxide of a specified cation along with the reaction product AD.
The term xe2x80x9cwater insoluble reaction product precipitatexe2x80x9d is intended to mean that the precipitate is substantially insoluble in water as will be understood by those skilled in the art. The solubility product constant in water at 25xc2x0 C. should generally be less than Ksp=10xe2x88x926, and preferably less than Ksp=10xe2x88x928.
The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The FIGURE is for illustration purposes only and is not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying FIGURE in which;
The FIGURE shows a schematic illustration of an apparatus used to make a mirror of the invention.